DISSECTION OP THE PELVIS. 361 



branches of the internal iliac artery (Plate 48). Thereafter he is to 

 examine the pyriformis and obturator internus muscles. 



The LuMBO-SACRAL Plexus (Plate 48) is composed of the anastomos- 

 ing nerve trunks for the supply of the hind limb. It is formed by the 

 inferior primary branches of the last three lumbar (4th, 5th, and 6th) 

 and first two sacral nerves, and it receives also a fasciculus from the 

 corresponding branch of the 3rd lumbar nerve. Each of these roots 

 emerges from the intervertebral foramen behind the vertebra after 

 which it is named ; thus, the root from the 6th lumbar nerve emerges 

 by the intervertebral foramen behind the 6th lumbar vertebra, the 1st 

 sacral root by the 1st inferior sacral foramen, and so on. The branches 

 of the plexus, taken in order from before to behind, are as follows : — 



1. Iliaco-muscular Branches, for the psoas and iliacus muscles. 

 Two of these are seen in Plate 48, one coming from the anterior root 

 of the plexus, and the other from the anterior crural nerve. 



2. The Anterior or Great Crural Nerve. In point of size, this 

 is the second nerve of the plexus. It derives its fibres from the first 

 two roots of the plexus (4th and 5th lumbar), and from the fasciculus 

 furnished by the 3rd lumbar nerve. 



3. The Obturator Nerve derives its fibres from the 4th and 5th 

 lumbar roots of the plexus. It descends in company with the obturator 

 vessels, resting on the pelvic surface of the ilium. Under cover of the 

 obturator internus muscle, it passes through the obturator foramen and 

 reaches the thigh. 



The 5th lumbar root, having given a branch to aid in the formation 

 of the anterior crural, and another to the obturator nerve, is continued 

 obliquely backwards between the internal iliac artery and the spine, to 

 join a broad nervous fasciculus to which the remaining roots of the 

 plexus (6th lumbar and first two sacral) contribute the whole of their 

 fibres. The remaining branches of the plexus are divisions of this 

 fasciculus. 



4. The Anterior Gluteal Nerves. Three or four in number, these 

 leave the pelvis and reach the hip by passing through the forepart of 

 the great sciatic opening, with the gluteal vessels. 



5. The Great Sciatic Nerve, the largest in the body, passes out into 

 the hip through the great sciatic foramen, behind the preceding. 



6. The Posterior Gluteal Nerves, distinguished as superior and 

 inferior, pass out behind the great sciatic. 



The 3ed Sacral Nbrvb. The inferior primary branch of this nerve 

 is continued as the internal pudic nerve, after giving a bundle of fibres 

 to aid in the formation of the haemorrhoidal nerve. 



The 4th Saoral Nerve receives the before-mentioned branch from 

 the 3rd nerve, and is continued as the haemorrhoidal nerve. 



The 5th Sacral Nerve gives a backward twig to the 1st coccygeal 



